RU2764773C2 - Method for producing argon and nitrogen - Google Patents

Abstract

FIELD: chemical processes.

SUBSTANCE: invention can be used in the chemical industry. In order to produce argon and nitrogen, process gas 22 containing NOx is subjected to the NOx absorption stage in the absorption tool 23, producing nitric acid 24 and tail gas 25 containing nitrogen, argon, and residual NOx. The tail gas 25 is then subjected to processing including at least one stage of removal of NOx, producing purified tail gas 26. At least part 26b of the purified tail gas is then subjected to the process of separation, producing a first stream 40 of a product containing argon and a second stream 37 of a product containing nitrogen. Proposed is a unit for producing argon and nitrogen.

EFFECT: group of inventions provides a possibility of reducing the material and energy costs and facilitating production of high-purity argon and nitrogen.

16 cl, 2 dwg, 1 ex

Description

Technical field

The invention relates to the production of argon and nitrogen.

State of the art

Argon (Ar) is a noble gas and is therefore chemically inactive. Due to this property, this gas is widely used in industry, for example, to create an inert atmosphere.

Molecular nitrogen (N ₂ ), due to its low reactivity, is also well suited for creating an inert atmosphere in various industrial and technological processes. Large quantities of nitrogen are also used in cryogenic engineering, but its main use remains the synthesis of ammonia, from which fertilizers, polymers, explosives and dyes are obtained.

Argon and nitrogen are usually produced by an air separation process, along with oxygen.

Most air fractionation plants use a process of fractional distillation of liquid air. This process is a cryogenic process in which the separation of argon, nitrogen and oxygen is essentially carried out using their different boiling points of -186°C, -196°C and -183°C, respectively. Known methods for liquefying air are, for example, the Linde cycle and the Claude cycle. Fractional distillation is usually carried out in systems having several distillation columns, usually three such columns.

One difficulty with this technology is that the boiling points of argon and oxygen are very close, making it difficult to obtain high purity argon by separating it from oxygen. The nitrogen produced in this way also usually contains argon and oxygen at the ppm level, which is also undesirable.

To maximize the degree of separation of argon and nitrogen, large distillation columns with a large number of trays can be used, the number of columns can be increased, or absorbent beds can be installed at the outlet of the columns, performing further purification of the argon and nitrogen streams obtained after distillation. All of these solutions, however, are costly in terms of plant design and process energy consumption.

In other plants for the production of argon and nitrogen, selective adsorption on membranes is used. However, these installations are not widely used and are expensive.

Disclosure of invention

It is an object of the present invention to provide a process which makes it possible to obtain substantially pure argon and nitrogen, and which is simple and low cost.

The applicant has found that the tail gas of the nitric acid synthesis process, due to its composition, is a suitable source for the production of argon and nitrogen.

It is known that the synthesis of nitric acid according to the Ostwald process includes the step of absorbing nitrogen oxides (NOx) in water, resulting in a stream of nitric acid and a tail gas containing nitrogen, argon, residual NOx and, optionally, also N ₂ O. In existing technologies , this tail gas is usually treated to remove NOx and possibly N ₂ O, in accordance with established air emission limits, and then removed. The invention proposes to treat at least a portion of this tail gas to separate the argon and nitrogen contained therein to obtain a product of high industrial value.

The above tasks are solved by the method in accordance with paragraph 1 of the formula, including the following steps:

subjecting the process gas containing NOx to NOx absorption steps in suitable absorption means to produce nitric acid and a tail gas containing nitrogen, argon and residual NOx;

subjecting the tail gas to a treatment including at least one NOx removal step to obtain a purified tail gas;

subjecting at least a portion of the purified tail gas to a separation process to obtain a first product stream containing argon and a second product stream containing nitrogen.

The absorption medium used in the NOx absorption step is preferably water.

Preferably, the cleaned tail gas is separated into at least two parts, the first part being subjected to the aforementioned separation process and the second part preferably being processed in a suitable expansion device (hereinafter referred to as "expander").

The argon concentration in the first product stream, as a result of the separation process, is greater than the argon concentration in the cleaned tail gas. Likewise, the second product stream has a nitrogen concentration greater than that of the scrubbed tail gas.

Preferably, the first product stream contains at least 99.5 vol% argon, more preferably at least 99.95 vol%, and even more preferably at least 99.995 vol%.

Preferably, the second product stream contains at least 99.5 vol% nitrogen, more preferably at least 99.95 vol%, and even more preferably at least 99.995 vol%.

The separation process is suitable for selectively separating at least argon from nitrogen. Preferably, the separation process is cryogenic treatment at a temperature not exceeding 133 K (-140°C).

In a preferred embodiment, the process gas is obtained by oxidizing an ammonia stream in the presence of air or enriched air. As a result, the source of nitrogen and argon contained in the tail gas is predominantly air or enriched air introduced in the oxidation stage.

The oxidation step generally comprises a first catalytic ammonia oxidation step to produce nitrogen monoxide NO and, to a lesser extent, nitrous oxide N ₂ O, and a second NO oxidation step to produce nitrogen dioxide NO ₂ or nitrogen tetroxide N ₂ O ₄ .

According to common practice in the industry, the compounds NO, NO ₂ and N ₂ O _{4 are} denoted by the general formula NOx.

The process gas absorption step is preferably carried out in an absorption column where the NOx contained in the process gas is at least partially absorbed in said absorption means, preferably water, to produce nitric acid and the aforementioned tail gas.

Optionally, prior to the absorption step, the process gas may be subjected to N ₂ O removal (so-called "secondary N ₂ O removal"). In some embodiments, N ₂ O is removed from the tail gas (so-called "tertiary removal"); in some embodiments, secondary removal and subsequent tertiary removal are provided.

The cleaned tail gas mainly contains nitrogen. Preferably, this gas contains nitrogen in an amount equal to or greater than 80% by volume, preferably greater than 90% by volume, and even more preferably from 95 to 98% by volume.

The cleaned tail gas also contains a significant amount of argon, typically at least 0.9 vol.%, preferably at least 1.0 vol.%, and even more preferably at least 1.1 vol.%.

This purified tail gas may also contain small amounts of water, preferably not more than 0.5% by volume, more preferably from 0.2 to 0.3% by volume.

This purified tail gas preferably contains negligible amounts of NOx and N ₂ O.

Preferably, the cleaned tail gas contains NOx in an amount of no more than 200 ppm, more preferably no more than 30 ppm, even more preferably no more than 5 ppm.

Preferably, the cleaned tail gas contains N2O in an amount of no more than 1000 ppm, preferably no more than 100 ppm, more preferably no more than 30 ppm, and even more preferably no more than 10 ppm.

Considering the property of NOx and N ₂ O to freeze during the separation process, the presence of NOx and N ₂ O in quantities exceeding the above will lead to a number of technological problems in the operation of the corresponding plant and safety problems during equipment shutdown, due to release to the atmosphere accumulated volumes of NOx and N ₂ O.

In some embodiments, the cleaned tail gas contains some oxygen, preferably no more than 5% by volume, more preferably 2 to 3% by volume.

The tail gas treatment preferably includes a DeNOx step via NOx catalytic reduction, more preferably selective catalytic reduction (SCR) in the presence of a reducing agent, preferably ammonia.

In other embodiments, the tail gas treatment includes a non-selective catalytic reduction (NSCR) DeNOx step. In this case, the cleaned tail gas is essentially free of oxygen and may contain traces of hydrocarbons or hydrogen, CO, CO ₂ and ammonia.

Preferably, the cleaned tail gas is at a pressure of more than 4 bar, preferably between 4 and 15 bar. This pressure corresponds to the preferred pressure for tail gas treatment.

The cleaned tail gas contains very little or no carbon dioxide. For example, the cleaned tail gas contains no more than 800 ppm CO ₂ , preferably no more than 700 ppm, and more preferably no more than 600 ppm. The designation ppm indicates the number of parts per million by volume.

Said separation process preferably comprises fractional distillation using different boiling points of -186°C for argon, -196°C for nitrogen and -183°C for oxygen (at standard temperature 288.15 K and pressure 101.325 kPa - normal conditions ).

Preferably, the separation process includes: cooling the purified tail gas, followed by expansion, achieving partial liquefaction and fractional distillation of the liquefied portion. Therefore, this process preferably includes fractional distillation of at least argon or nitrogen or oxygen at an appropriate boiling point.

The process may include a step of removing CO ₂ before cryogenic treatment in order to avoid freezing and accumulation of CO ₂ in the cryogenic block.

The removal of CO ₂ preferably includes passing the gas through a molecular sieve.

Another feature of the invention is a process for producing an argon-containing stream and a nitrogen-containing stream through a process for separating a purified tail gas from a nitric acid synthesis plant, wherein the purified tail gas is obtained using the following steps:

subjecting the process gas containing NOx to NOx absorption steps in suitable absorption means to produce nitric acid and a tail gas containing nitrogen, argon and residual NOx;

subjecting the tail gas to a treatment including at least one NOx removal step to obtain a purified tail gas.

Another feature of the invention relates to an argon and nitrogen production plant according to the claims.

The cleaned tail gas has a higher argon content and a significantly lower oxygen content compared to air. For this reason, obtaining argon from such a purified tail gas is much easier and more profitable than isolating it from air. In particular, a lower oxygen content (or no oxygen in the case of NSCR non-selective catalytic reduction) helps to obtain an argon-containing product stream, since oxygen, due to its close boiling point, is the most difficult component to separate from argon.

Another advantage of using tail gas is the low content of pollutants (in particular NOx and N ₂ O), which allows to obtain streams of argon and nitrogen of high purity and ensures the correct functioning of the plant, eliminating the need to deal with problems associated with N ₂ O and toxic gases, such as NOx .

Another advantage is that the cleaned tail gas is under high pressure (typically more than 4 bar, eg 4-15 bar), which allows the gas to be partially liquefied by its expansion. This no longer requires a dedicated compressor for this tail gas, which is an economic advantage over a conventional air separation plant, since the compressor is the most expensive component.

Given the absence of a compressor due to the fact that the tail gas is under pressure, and the simpler arrangement of distillation columns due to the reduced oxygen content in the tail gas, the capital cost of a fractionation plant in accordance with the invention is significantly lower than for a conventional air separation plant. This will result in competitive prices for the production of argon and nitrogen.

This is also confirmed by the fact that for the nitric acid plant it is necessary to purchase electricity from the market to compensate for the loss of released power due to the fact that the tail gas is not fully expanded in the expander, but is subjected to fractionation.

For these reasons, purified tail gas is a particularly suitable source for the production of argon and nitrogen.

Moreover, the invention increases the value of the tail gas emitted by the plant due to the production of nitric acid, due to the fact that in existing technologies this gas was simply released into the atmosphere. The invention thus creates an important source of income for nitric acid synthesis plants. Therefore, one feature of the present invention is the co-production of nitric acid, argon and nitrogen. The nitrogen thus obtained can, for example, be sold on the market or used to increase productivity in a possible combination of an ammonia plant and a nitric acid plant.

Another advantage of the invention is the saving of natural resources and energy compared to existing technologies for the production of argon and nitrogen, which use raw materials (ie distillation or selective adsorption), and which require significant energy costs.

The invention is particularly attractive when the local need for argon and nitrogen (where there is a plant for the synthesis of nitric acid) is not balanced with the composition of the air.

The advantages of the invention will become more apparent upon consideration of the following detailed description relating to the preferred embodiments of the invention.

Brief description of the drawings

Below the invention is discussed in more detail with reference to the accompanying drawings, in which:

in fig. 1 is a simplified block diagram of an installation according to the invention;

in fig. 2 is a block diagram of a plant for the co-production of nitric acid, argon and nitrogen in accordance with a preferred embodiment of the invention.

Detailed description of the invention

The installation shown in Fig. 1 mainly includes an absorption column 4, a unit 6 for cleaning the tail gas leaving this column, an expander (expansion device) 7 and a section 2 for separating argon streams and a nitrogen stream.

This setup works as follows.

Process gas 22 containing NOx and, to a lesser extent, N ₂ O, and a water stream 23 are fed into the absorption column 4. Inside the column 4, NOx is partially absorbed in water to form a stream 24 containing nitric acid and a tail gas 25 containing, mainly nitrogen and small amounts of oxygen, argon, water, N ₂ O and residual amounts of NOx.

The tail gas 25 is sent to a purification unit 6 where NOx and optionally also N ₂ O are at least partially removed to form a purified tail gas 26. The gas 26 exiting the purification unit 6 is preferably at a pressure of 4 to 15 bar.

This purified gas 26 is preferably divided into two parts: the first part 26a is expanded in the expander 7 and the atmosphere is vented with stream 27, and the second part 26b is fed into section 2 and subjected to a separation process, obtaining stream 40 containing argon and stream 37, containing nitrogen.

In FIG. 2, the plant shown in FIG. 1 is shown in more detail. It includes, in particular, section 1 for the synthesis of nitric acid and section 2 for the production of argon and nitrogen.

Section 1 mainly includes a reactor 3 for the catalytic oxidation of ammonia, an absorption column 4, a heat exchanger 5, a block 6 for removing NOx and, optionally, removing N ₂ O, and an expander 7. In particular, in the case of high-performance plants, section 1 also contains a compressor between the reactor 3 and the absorption column 4.

Section 1 works as follows.

Ammonia stream 20 and air stream 21 are fed into reactor 3. Within reactor 3, ammonia is catalytically oxidized to form nitrogen monoxide NO and, to a lesser extent, nitrous oxide N ₂ O, wherein at least a portion of the NO is further oxidized to form nitrogen dioxide NO ₂ or nitrogen tetroxide N ₂ O ₄ to form gas stream 22.

This gas stream 22 and water stream 23 are introduced into an absorption column 4 where NOx is at least partially absorbed to produce nitric acid 24.

The absorption column 4 also produces a tail gas 25 as an overhead product, mainly containing nitrogen and, to a lesser extent, oxygen, water, argon N ₂ O and residual NOx.

The tail gas 25 is preheated in the heat exchanger 5 and then fed to the block 6. As shown in FIG. 2 example, block 6 contains a DeNOx section within which NOx is at least partially removed by selective catalytic reduction (SCR).

Block 6 operates at a pressure of 4 to 15 bar and provides gas 26 containing mainly nitrogen, 2-3% oxygen, 0.2-0.35% water, NOx <30 ppm and N ₂ OOO ppm

This gas 26 is separated into two parts: the first part 26a is expanded in the expander 7, and the second part 26b is removed from section 1 for the synthesis of nitric acid and fed into section 2 for the production of argon and nitrogen.

The expander 7 generates at least a portion of the power required for the compressors (not shown) of the nitric acid section 1. The expanded gas 27 is vented to the atmosphere.

Section 2 for producing argon and nitrogen mainly contains a heat exchanger 8, an expander 9, a separator 10 and a distillation device 11.

According to the example in FIG. 2, this device 11 includes: a first distillation column 12 operating at a pressure of about 4-5 bar, a second distillation column 13 operating at atmospheric pressure and a third distillation column 14 separating argon.

Section 2 works as follows.

Part 26b of the gas exiting section 1 is mixed with recycle stream 32 and fed to heat exchanger 8 where it is cooled, giving off heat to stream 31 exiting separator 10, to produce cooled gas 28.

The cooled gas 28 is then sent to expander 9 where it is partially liquefied. The expander 9 is a valve or a turbine, depending on the embodiment.

The partially liquefied gas 29 is fed into a separator 10. In the separator 10, the liquid phase 30 is separated from the gas phase 31. The liquid phase 30 is sent to the distillation device 11, while the gas phase 31 is sent to the heat exchanger 8 to cool the incoming gas 26b, after which it is returned into a closed loop in the form of a stream 32.

In more detail, the liquid phase 30 enters the first column 12 in which the separated nitrogen gas exits at the top and the liquid fraction 34 containing nitrogen, oxygen and argon exits at the bottom.

Liquid fraction 34 is sent to the second column 13, and nitrogen 33 enters the condenser 15, where it condenses, exchanging heat with the tail fraction 35 of column 13.

According to the example shown in FIG. 1, the condensed nitrogen stream 36 leaving the condenser 15 is divided into two parts: the first part 36a is sent to the second column 13, and the second part 36b is sent to the first column 12 as a return flow.

In the second column 13, nitrogen 37 and oxygen 38 are separated.

Fraction 39, containing argon and oxygen, is collected at an intermediate point of the second column 13 and sent to the third column 14, in which practically pure argon 40 and oxygen 41 are separated.

Example

A plant producing 500 mt/d (metric tons per day) of nitric acid produces a process gas containing 5-6% NOx at the inlet of the absorption column. The tail gas at the outlet of this column contains approximately 300-500 ppm NOx, and at the outlet of the purification section (section of catalytic reduction - SCR) of this gas contains approximately 0-22 ppm. Subjecting to this treatment in the separating section, about 77,000 kg/h of nitrogen and about 1300 kg/h of argon are obtained.

Claims (25)

1. A method for producing argon and nitrogen, in which subjecting the process gas (22) containing NOx to an NOx absorption step in the absorption means (23) to produce nitric acid (24) and a tail gas (25) containing nitrogen, argon and residual NOx; subjecting the tail gas (25) to a treatment including at least one NOx removal step to obtain a purified tail gas (26); subjecting at least a portion (26b) of the purified tail gas to a separation process to obtain a first product stream (40) containing argon and a second product stream (37) containing nitrogen. 2. Process according to claim 1, wherein the argon content of the first product stream (40) is at least 99.5 vol%, more preferably at least 99.95 vol%, and even more preferably at least least 99.995 vol.%. 3. Process according to claim 1 or 2, wherein the nitrogen content of the second product stream (37) is at least 99.5% by volume, more preferably at least 99.95% by volume, and even more preferably at least 99.995 vol.%. 4. The method according to any one of the preceding claims, wherein the argon content in the purified tail gas (26) is at least 0.9 vol.%, preferably at least 1.0 vol.%, even more preferably at least 1.1 vol.%. 5. The method according to any of the preceding claims, wherein the NOx content of the purified tail gas (26) does not exceed 200 ppm, preferably does not exceed 30 ppm, more preferably does not exceed 5 ppm. 6. The method according to any one of the preceding claims, wherein the N ₂ O content in the purified tail gas (26) does not exceed 1000 ppm, preferably does not exceed 100 ppm, more preferably does not exceed 30 ppm, even more preferably does not exceed 10 ppm . 7. The method according to any one of the preceding claims, wherein the oxygen content of the purified tail gas (26) preferably does not exceed 5 vol%, more preferably 2 to 3 vol%. 8. Process according to any one of the preceding claims, wherein the pressure of the purified tail gas (26) is greater than 4 bar, preferably between 4 and 15 bar. 9. Process according to any one of the preceding claims, wherein the process gas (22) containing NOx is obtained by oxidizing an ammonia stream (20) in the presence of air or enriched air (21). 10. A process according to any one of the preceding claims, wherein the purified tail gas contains not more than 800 ppm CO ₂ . 11. The method according to any one of the preceding claims, wherein the separation process is cryogenic treatment. 12. The method according to claim 11, in which, during separation, the purified tail gas (26) is cooled with its subsequent expansion, partial liquefaction is carried out and the liquefied part (30) is subjected to fractional distillation. 13. The method according to claim 11 or 12, including the step of removing CO ₂ before cryogenic treatment, preferably by means of a molecular sieve. 14. Installation for the production of argon and nitrogen, including absorption column (4) configured to produce process gas (22) containing NOx and absorb NOx in the respective absorption means (23) to produce nitric acid (24) and tail gas (25) containing nitrogen, argon and residual NOx ; a tail gas purification unit (6) (25) suitable for removing NOx and obtaining purified tail gas (26); separation section (2) suitable for separating the first stream (40) of the product containing argon, and the second stream (37) of the product containing nitrogen, and supply to the separation section (2) at least part (26b) of the purified tail gas ( 26). 15. Plant according to claim 14, in which the separation section (2) includes a heat exchanger (8) suitable for cooling at least one part (26b) of the purified tail gas, to obtain a cooled gas (28); an expansion device (9) for the cooled gas (28), providing a partially liquefied gas (29); a separator (10) in which the liquefied fraction (30) of the partially cooled gas (29) is separated from the fraction (31) that has not been liquefied; a distillation device (11) configured to receive a liquefied fraction (30) and separate an argon-containing stream (40) and a nitrogen-containing stream (37). 16. Plant according to claim 14 or 15, comprising a reactor (3) for oxidizing the ammonia stream (20) in the presence of air or enriched air (21) to produce process gas (22) containing NOx.

Images